Conventionally, tin plating or tin-lead alloy plating has been performed on components to be soldered, for instance components which constitute electronic equipment such as chip parts, crystal oscillators, bumps, connector pins, lead frames, various hoop materials, lead pins of packages and circuits of printed boards. The pure tin plating has problems such as deterioration in solderability and occurrence of whisker-shaped crystals (whiskers) on a film. Furthermore, the use of lead has recently been reduced from a standpoint of environmental protection; thus, lead-free plating has become desirable.
As the lead-free plating, for example, a tin-silver alloy, a tin-bismuth alloy, a tin-copper alloy and the like have been considered. However, there are some drawbacks that a plating bath of the tin-silver alloy easily becomes unstable and a plating film of the tin-bismuth alloy tends to generate cracks.
On the other hand, the tin-copper alloy hardly generates cracks and has superior joining strength but has a drawback that a melting point is high. In tin-copper alloy plating, a plating bath containing certain organic compounds and also a tin-silver-copper alloy plating bath are disclosed in, e.g. Japanese Patent Publication Laid-Open No. 2001-164396 (hereinafter referred to as Patent Document 1).
Moreover, a solder material used for a board to be joined with a plating film is also desired to be lead-free. For instance, the tin-silver-copper alloy having a composition close to a eutectic point has been in-use as the solder material as disclosed in Japanese Patent Publication Laid-Open No.H5-50286 and U.S. Pat. No. 5,527,628 (hereinafter referred to as Patent Documents 2 and 3, respectively). If a plating film having a composition similar to that of the solder material of the board is used, it is expected to lower energy cost at the time of joining and to improve joinability. Besides the aforementioned Patent Document 1, some results related to a tin-silver-copper plating solution and production of a plating film using the plating solution are disclosed, e.g. in “Electrocrystallization of Sn-0.7Cu-0.3Ag Ternary Alloy from Methanesulfonic Acid Bath and Effect of T3HPP on Bath Stability” (hereinafter referred to as Non-patent Document 1) in the Resume of the 103rd Symposium of the Surface Finishing Society of Japan, page 54, disclosed on Mar. 5, 2001, and also in “Sn—Ag—Cu Alloy Plating for Lead-Free Solder” (hereinafter referred to as Non-patent Document 2) in the Resume of the 103rd Symposium of the Surface Finishing Society of Japan, page 58, disclosed on Mar. 5, 2001.
However, the compositions of the plating baths disclosed specifically in Patent Document 1, Non-patent Documents 1 and 2 are disadvantageous in that each metal concentration is low or out of balance. The plating baths have a further problem with productivity for using in industrial plating because of instability of the plating solutions. In addition, although the tin-silver-copper alloy-plating bath is superior to the tin-copper alloy-plating bath in solder wettability, solution stability and soldering strength, the preparation thereof is difficult because it contains three kinds of metal ions and complexes thereof. On an industrial scale, particularly, an amount of chemical liquid handled is so large and the bath is used for such a long time that solids may occur or characteristic of the solution may change during plating processes, which causes problems such as unstable plating quality and so on. Moreover, especially in the case of producing a plating solution having a high metal-ion concentration, there are following problems. If a soluble anode having 90 percent or more of tin is used, silver ions tend to precipitate as silver compounds or metal silver by substitution with tin of the soluble anode, which deteriorates stability of the solution. On the other hand, if an insoluble electrode is used, insoluble precipitates are generated as Sn2+ are oxidized to Sn4+ around the insoluble electrode, which degrades the quality of the solution.
Furthermore, a conventionally known lead-free plating film has a melting point higher than that of a tin-lead plating film by about 20° C. Thus, a lead-free plating film having a lower melting point has been desired to reduce the energy cost at the time of joining and to reduce an amount of heat applied to other components.
Moreover, the solder material used for the board to be joined with the plating film is also desired to be lead-free, and for example, the tin-silver-copper alloy having the approximate eutectic composition has been in-use as the solder material. However, a plating film having a composition similar to that of the tin-silver-copper alloy has not been obtained due to the difficulty in producing the plating solution as described above. Consequently, the plating film which has good joinability with the solder material using the tin-silver-copper alloy has been desired.
An object of the present invention is to provide the tin-silver-copper plating solution having less solid precipitations and better storage stability over a long time. Another object of the present invention is to provide an electrolytic plating method of the tin-silver-copper plating solution whereby quality deterioration hardly occurs even if a plating process is continuously performed in a large amount. The electrolytic plating method also makes it possible that a plating process can be carried out with a higher current density. Furthermore, the present invention collaterally aims to provide the plating film having a low melting point, less cracks and better joining strength and a soldering method using the plating film whereby the mounting can be carried out at a lower heating temperature.